xfs: don't allow negative error tags

Errors go from zero which means no error to XFS_ERRTAG_MAX (22).  My
static checker complains that xfs_errortag_add() puts an upper bound on
this but not a lower bound.  Let's fix it by making it unsigned.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>